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Anytime you turn on your TV or check a newspaper you’re likely to be assaulted by ads from the four major wireless carriers claiming that they provide the fastest and most reliable coverage. They can’t all be the best, and few of us believe all the advertising hype, but that doesn’t mean there’s no way to find out which carrier (AT&T, Sprint, Verizon or T-Mobile) comes the closest to delivering on its claims.

Overall Scores

RootMetrics, a Seattle-based testing company, tested service in 125 metro areas and all 50 states, making thousands of phone calls that resulted in more than 4.6 million data points. The calls were not made by machines; they were made by humans using off-the-shelf phones, the company says. RootMetrics tested for things like the ease of making a data connection, call quality, download speeds, and the ability to send a text. Overall the winner was Verizon, followed closely by AT&T. Sprint was a relatively distant third, while T-Mobile brought up the rear. Here’s how RootMetrics summed up the results:

“It’s clearly a two-network race when comparing results at the national scale, with Verizon and AT&T head and shoulders above the other networks. The separation between these two national heavyweights was small, with most categories showing a neck-and-neck race. At the end of the day, however, the numbers don’t lie: Verizon owns national bragging rights in our RootScore Awards, sweeping our Overall, Call, Data, and Text categories. With vast LTE coverage and consistently strong performances across all of our testing, Verizon also finished on top in our Reliability Index, making Verizon the nation’s most reliable network in our national results."

When it comes to data speed, AT&T won by a nose, just a bit faster than Verizon’s network. T-Mobile, which has boasted that it offers the fastest 4G service, was well behind Verizon, and Sprint was in the rear. The tests included and averaged in 3G connections, along with faster 4G service depending on which was available at each test location.

AT&T’s advantage on speed was strongest in metro areas; of the 125 urban markets tested by RootMetrics, AT&T won or tied for first in 92.

I was very skeptical of T-Mobile’s recent claim that it offered the fastest network because it was based on a survey of customers, not on independent testing -- and the RootMetrics report shows that my doubts were well founded. Still, T-Mobile does well in some cities:

“While the ‘uncarrier’ is starting to offer some truly noteworthy speeds at the metro level, its performance drops dramatically at the broader state and national levels. If you are in an urban environment, T-Mobile could be worth a look; but outside of metros, the story is different,” says RootMetrics.

As always, I urge you to test service in your home or office before selecting a carrier. If you’re thinking of going with Verizon, for example, have a friend who subscribes to that service come over and see how well his or her smartphone does at your location. Service within cities often varies from neighborhood to neighborhood and even from block to block.

San Francisco journalist Bill Snyder writes frequently about business and technology. He writes the Tech's Bottom Line blog for InfoWorld, and his work appears regularly in CIO.com and the publications of Stanford's Graduate School of Business and the Haas School of Business at the University of California at Berkeley.